gp140 is the Epstein-Barr virus receptor and the C3d receptor (EBVR/C3dR) of human B lymphocytes. Recently, we have shown that cross-linking of EBVR/C3dR on cell surface by polyclonal anti-gp140 induced B cell activation, in presence of T cell factors. Immunoregulatory abnormalities of EBV-induced B cell activation have been demonstrated in rheumatoid arthritis (RA) patients. These data prompted us to analyze the putative presence of anti-EBVR/C3dR autoantibodies in human sera. The IgG fractions from eleven RA and 10 normal sera were tested for their ability to: (a) bind to Raji cell surface; (b) inhibit the binding to cell surface of 3 anti-EBVR/C3dR monoclonal antibodies (mAb), which recognized different epitopes on gp140; (c) inhibit the binding of particle-bound C3d and (d) react with 1% Nonidet-P40-solubilized gp140 from Raji cell membranes, in immunoblotting assays. Three RA sera carry anti-EBVR/C3dR autoantibodies which react with gp140 expressed on Raji cell surface or its solubilized form. The purification of monomeric IgG fraction of selected RA sera ruled out involvement of immune complexes carrying C3 molecules, which could interfere in these assays. One of these 3 RA sera was able to inhibit the binding to cell surface of anti-EBVR/C3dR mAb and particle-bound C3d. However, the 2 other RA sera, found positive by immunoblotting, did not inhibit particle-bound C3d and presented differences in their inhibitory effect on anti-EBVR/C3dR mAb binding to Raji cell surface. These data allow us to demonstrate differences which exist in the properties of anti-EBVR/C3dR autoantibodies. These autoantibodies were not detected in all the normal and other RA sera. Anti-EBVR/C3dR autoantibodies could play a role "in vivo" in B lymphocyte activation of RA patients.